The invention relates to superconducting members and methods of manufacture thereof.
By superconducting member is meant a member which will exhibit superconductivity when its temperature is lowered below its critical temperature. Materials of particular interest in this field are those which have comparatively high critical temperatures and comparatively high critical magnetic fields. Such materials are compounds of A15 crystal structure having the general formula A.sub.3 B where A comprises niobium or vanadium and B comprises one or more of the elements aluminium, gallium, indium, silicon, germanium and tin.
Patent Specification No. 52623/69 described a method of manufacturing such a superconducting member which permits formation of the desired final size and shape of superconducting member by a simple mechanical forming operation applied simultaneously to all the components which go into the manufacture. In particular, the invention of Patent Specification No. 52623/69 provides a method of manufacturing a superconducting member wherein there is formed an alloy essentially consisting of a carrier material and at least one element from the group consisting of aluminium, gallium, indium, silicon, germanium and tin, and the alloy is contacted with a base material essentially consisting of niobium or vanadium and heat treated to cause a solid state reaction between the niobium or vanadium and the element or elements from the said group to form a superconducting compound therewith, the carrier material being such as will not react substantially with the base material under the heat treatment and the heat treatment temperature being controlled for avoiding melting of the alloy at any stage during the reaction.
The present invention is concerned with the stabilisation of superconducting members manufactured by this method.
By stabilisation is meant the prevention or amelioration of the undesirable effects of sudden movement of magnetic flux, known as `flux jumps`, within the superconducting member. The invention relates to the combination in the same superconducting member of two methods of stabilisation known respectively as `filamentary stabilisation` in which the superconducting material is present in the form of many fine filaments in a normally-conducting matrix, and `dynamic stabilisation` in which a proportion of the normally-conducting matrix is in the form of a material of high electrical conductivity such as pure copper.
The method described in Patent Specification No. 52623/69, and referred to above, is particularly suitable for producing many fine filaments of superconductors in a matrix of normal material. However, the normal material thus formed is an alloy and not a pure metal. In general it is not practicable to so adjust the starting compositions and the reaction conditions that the alloy matrix will have the required level of electrical conductivity for dynamic stabilisation (resistivity about 10.sup.-8 chm cm).
With ductile filamentary superconductors, e.g. niobium titanium alloys, it is possible to fabricate the superconductors embedded in a pure copper matrix since no significant interdiffusion occurs at the heat treatment temperatures necessary to get optimum critical currents (300.degree.-400.degree. C). However, with A15 superconductors produced by the abovedescribed method, the B element from the composite material will diffuse rapidly into the copper. This has the disadvantages of slowing the formation of A.sub.3 B and of producing Kirkendall porosity in the composite material, but its most deleterious effect is to raise the resistance of the copper to an unacceptably high value for D.C. stabilisation.
One method of avoiding this diffusion problem would be to apply the pure metal (e.g. copper) to the composite material after the A.sub.3 B reaction by some process which can occur at room or low temperatures, e.g. by electrodeposition. However, electrodeposits are known to have high resistivities which are not removed on low temperature annealing. There are also many potential applications of multifilament superconductors where the wires are required to be bent more sharply than is possible with reacted material. In these cases it would be possible to react the composite after the coil has been wound if the copper, or other pure metal, can be protected from diffusion of impurities.